Land-based communication systems increasingly use optical fibers as the transmission medium. One important reason is the wide bandwidth provided by optical fibers. The simplest fiber optic systems use a single laser and a single photodetector at opposite ends of the fiber. The laser is modulated by the data signal and the photodetector detects the data-modulated envelope of the laser carrier frequency. That is, the optical fiber is being used for single-carrier transmission. However, the electronics for the transmitters and receivers are generally limited to frequencies of a few gigahertz and below. While gigabit per second data rates are significant improvements over rates available by coaxial systems or radio-broadcast systems, such single-carrier transmission does not fully utilize the bandwidth of the fiber, often the most expensive component of a long-haul communication system.
Fiber optics also are finding use in local-area networks (LANs) and local distribution systems. One of the problems of such systems has been the difficulty of switching an optical-carrier signal between the sender and a selected receiver.
As a result of the limitations of single-carrier fiber optic communications systems, many proposals have been made for wavelength-division-multiplexing systems, as has been described by Ishio et al in a technical article entitled "Review and Status of Wavelength-Division-Multiplexing Technology and Its Application" appearing in Journal of Lightwave Technology, Vol. LT-2, 1984 at pages 448-463. A fundamental configuration for such a system is illustrated in FIG. 1 in which a transmission end 10 is linked to a receiving end 12 by an optical fiber 14. There are n channels of electronic inputs CH.sub.1 to CH.sub.n, each controlling a modulator 16 modulating a photoemitter 18, such as a laser diode. Each of the photoemitters 18 is emitting light at a frequency different from the other photoemitters 18. The different wavelengths of light from the n photoemitters 18 are combined in an optical multiplexer 20. The optical fiber receives the optical output of the multiplexer 20 and provides an optical input to an optical demultiplexer 22. The demultiplexer 22 separates its input into n optical outputs corresponding in frequency or wavelength to the photoemitters 18. The so separated light is detected by photodetectors 24, which may be broad band, and the electronic outputs are demodulated by receivers 26 into the n channels of received data. The demultiplexer is a generalized optical filter.
It is possible to use a single fiber 14 for bidirectional transmission as is explained both by Ishio et al and by Reichelt et al in a technical article entitled "Wavelength-Division Multi Demultiplexers for Two-Channel Single-Mode Transmission Systems" appearing in Journal of Lightwave Technology, Vol. LT-2, 1984 at pages 675-681.
A related system provides a passive hub for communications between n user stations. One such system is the LAMBDANET system described by Kobrinski in a technical article entitled "Applications of coherent optical communication in the network environment" appearing in Coherent Technology in Fiber Optic Systems, SPIE, Vol. 568, 1985 at pages 42-49 and by Kobrinski et al in a technical article entitled "Demonstration of high capacity in the LAMBDANET architecture: A multiwavelength optical network" appearing in Electronics Letters, Vol. 23, No. 16, 1987 at pages 824-826. As illustrated in FIG. 2, there are n nodes 30, each linked to an n x n star coupler 32 by an transmission optical fiber 34 and by a receiver optical fiber 36. Each node 30 transmits digital data modulated on an optical carrier having a different optical carrier frequency f.sub.1, f.sub.2 through f.sub.n. The frequencies are determined by different photoemitters 18 in each of the nodes 30. The star coupler 32 may be a passive device which consumes no electrical power and merely combines all its optical inputs into equivalent parallel outputs so that all the receiver fibers 36 carry all the optical signals from all the nodes 30.
The system described by Kobrinski et al relies on a diffraction grating for the demultiplexer. There were n optical fibers receiving the spectrally separated outputs of the diffraction grating and each working fiber had associated therewith a photodetector. The described system had a channel spacing of 2 nm. The diffraction grating optically filters the input signal to different angular positions dependent upon the carrier wavelength. The diffraction grating demultiplexer has the problem of relying on the physical arrangement of the fibers relative to the grating. The wavelengths of the transmitters are fixed relative to the receivers. Further, as the emitting lasers age, their frequencies change. Therefore, the fibers need to be moved relative to the grating if they are to stay tuned to the laser frequencies. There must be n receivers for n channels if frequent channel hopping is expected. If mechanical tuning is provided, there are severe reproducibility problems.
The two above described LAMBDANET systems rely on direct detection of the optical signal within the nodes 30, for example, a diffraction grating demultiplexer followed by photosensitive transistors. However, the LAMBDANET system can be modified to incorporate coherent detection based on optical heterodyning to select the desired channel, that is, the optical carrier frequency f.sub.i corresponding to the desired transmitting node 30. Optical heterodyning has been described in a technical article by Bachus et al entitled "Ten-channel coherent optical fibre transmission" appearing in Electronics Letters, Vol. 22, No. 19, 1986 at pages 1002 and 1003. A simplified coherent detection system is shown in FIG. 2. A tunable laser 38 emits a continuous beam at an optical local oscillator frequency f.sub.LO. An optical combiner 40 combines the optical signal received on the receiver fiber 36 and that received from the local oscillator 38. By proper tuning of the local oscillator frequency f.sub.LO, the difference frequency f.sub.i -f.sub.LO of the desired channel is brought within the frequency range of a detector 42 which is a low pass optical filter and a square law converter. The data signal carried by the selected carrier at frequency f.sub.i can thereafter be IF demodulated by the electronics 44.
The optical heterodyning of Bachus et al allows for very narrow channel spacing; 6 GHz or 0.013 nm was demonstrated. However, optical heterodyning requires a narrow-band, continuously tunable laser for the local oscillator 38. Bachus states that the laser must be stabilized in temperature to 0.001.degree. C. and in current to 1 .mu.A. In order to suppress phase noise, the laser linewidth must be very narrow. Needless to say, these requirements put severe demands on the design of the local oscillator laser.
As has been explained by Spencer in his review article on WDM systems entitled "State-of-the-art survey of multimode fiber optic wavelength division multiplexing" appearing in Proceedings of the SPIE, vol. 403, 1983 at pages 117-130, the demultiplexer presents one of the major technical challenges if the communication system is to carry a significant number of channels. Many of the demultiplexers described by Ishio et al and by Spencer are complex, as is the demultiplexer of the Bachus et al system. Except for the optical heterodyning, the number of channels is severely limited. Spencer teaches that few systems use optical filters as the wavelength discriminating device but they are instead used more for the purpose of improving signal to noise. The two main types of filters disclosed by Spencer in association with wavelength discrimination are interference filters and high or low pass dichroic filters.
One of the major drawbacks of the demultiplexers described by Spencer, by Kobrinski et al, and by Ishio et al is that the demultiplexed frequencies are physically fixed by the structure or at least the configuration of the demultiplexer. Tunability is mostly discussed in conjunction with angular changes in physical optics. It thus becomes very difficult to build one demultiplexer which can be tuned to different frequencies. Such a tunability would allow detection of different ones of the data channels. When tuning is achieved by physical movement of the diffraction grating or fibers, the tuning is necessarily slow. Furthermore, there is an inevitable amount of backlash in mechanically adjustable opital components, which reduces the accuracy of the optical tuning.
The tunable optical filters of the prior art offer different advantages and disadvantages, as discussed below.
The Fabry-Perot or tunable etalon type of filter is desirable in that it is not dependent upon the polarization of the light and has a relatively low insertion loss of 2 to 3 dB. Its finesse of 40 to 100 is acceptable. However, its mechanical tuning is slow and its repeatability of tuning is very poor. This type of filter can be cascaded into two stages. The finesse is then increased to about 1000 but the insertion loss is raised to above 10 dB. The mechanical tuning then becomes slower and more difficult to control.
An electro-optic type of optical filter relies on electronic tuning so that channel hopping can be done on the order of nanoseconds. The tuning range of about 10 nm is not very good but the repeatability and fine tunability are acceptable. The insertion loss of 4 to 6 dB is marginal. The biggest disadvantages of electro-optic filters are the small finesse of about 10 and the polarization dependency.
Semiconductor optical filters of the injection current type also provide nanosecond electronic tuning. Their channel spacing of about 0.6 nm is fairly narrow but the tuning range of about 4 nm is not very useful. They are repeatably tunable and offer fine tunability. However, they provide very small values of finesse, are polarization dependent and present large insertion losses of over 10 dB.